Internal combustion engine air/fuel ratio control is known in which fuel command magnitude is determined in response to an estimate of the magnitude of an operator-controlled engine inlet air rate. Such control may be termed "air-lead" control. If fuel is controlled to individual cylinders, such as through conventional port fuel injection, the corresponding air rate of the cylinders must be estimated and the fuel command determined in response thereto to provide a desirable air/fuel ratio to the cylinders.
A desirable engine air/fuel ratio may be the well-known stoichiometric air/fuel ratio. Efficient reduction of undesirable engine exhaust gas constituents through conventional catalytic treatment thereof occurs when the engine air/fuel ratio is the stoichiometric ratio. Even minor deviations away from the stoichiometric ratio can degrade emissions reduction efficiency significantly. Accordingly, it is important that the engine air/fuel ratio be closely controlled to the stoichiometric ratio.
The precision of the described air-lead control is limited by the precision of the cylinder inlet air rate sensing or estimation. When engine inlet air dynamics are in steady state, such that the air pressure in the engine intake manifold is substantially constant over a predetermined time period, precise cylinder inlet air rate sensing may be provided through use of a conventional mass airflow meter in the engine inlet air path. The absence of any significant manifold filling or depletion in steady state provides for a direct correspondence between manifold inlet air rate and cylinder inlet air rate. Accordingly, the airflow meter may alone be used for accurate cylinder inlet air rate estimation in steady state.
The airflow meter may not accurately characterize cylinder inlet air rate under transient conditions, such as conditions in which there is no direct correspondence between manifold inlet air rate and cylinder inlet air rate. This is primarily due to the significant time constant associated with manifold filling or depletion, and airflow meter lag. Transient conditions can arise rapidly during engine operation, such as by any substantial change in engine inlet throttle position TPOS, or by any other condition that perturbs manifold absolute pressure MAP. Any significant perturbation in steady state operating conditions will rapidly inject substantial error in the airflow meter estimate of cylinder inlet air rate. Accordingly, if a mass airflow meter is to be used for cylinder air rate estimation under steady state operation, some variation in the estimation approach is required to retain estimation accuracy when outside steady state operation. Necessarily, there must be a reliable determination of whether the engine is operating in steady state or under transient conditions.
Engine parameters such as engine intake manifold absolute pressure MAP and air inlet valve position TPOS may be used to categorize the air dynamics as steady state or transient. The lack of manifold filling or depletion that characterizes steady state air dynamics is directly indicated by a substantially steady MAP over a predetermined number of MAP samples. Such provides sufficient information with which to diagnose an entry into steady state. It has been proposed to use one criterion, such as the described substantially steady MAP criterion to detect or diagnose both entry into and exit from steady state. Two difficulties result from the use of a single criteria with which to transition into or out of steady state air dynamics. First, signal noise may trigger unnecessary transitions. Second, detection of transitions, especially out of steady state, may be delayed while waiting for detailed analyses, such as analyses designed to reduce sensitivity to noise, to come to a conclusion.
Signal noise may come from a sensor, such as a MAP or TPOS sensor, or may result from analog to digital signal conversion quantization effects. The noise may cause misleading variations in the interpreted signal, leading to false indications of MAP or TPOS variation, and thus to an improper diagnosis that the air dynamics are no longer in steady state. Such may reduce cylinder air rate estimation accuracy.
If detection of a transition is delayed, especially a transition out of steady state, cylinder inlet air rate estimation accuracy may be degraded. For example, a significant number of MAP or TPOS samples may be required to determine if indeed the manifold is not filling or depleting--indicating steady state operation. Once in steady state, mass airflow meter information may accurately characterize cylinder inlet air rate. However, a slight change in MAP or TPOS may quickly erode the accuracy of the characterization by rapidly leading to accumulation or depletion in the manifold. A cylinder inlet air rate estimation penalty is incurred during the period of time required for accumulation and interpretation of MAP or TPOS signals so as to diagnose the exit from steady state. Accordingly, the duration of such a time period should be minimized.
It therefore would be desirable to provide a characterization of engine inlet air dynamics that is substantially insensitive to signal noise and yet rapidly detects entry into or exit out of a steady state condition, so the appropriate cylinder air rate estimation approach may be applied at all times during engine operation, for precise engine air/fuel ratio control.